The Kjeldahl method and four classic spectrophotometric methods (Biuret, Lowry, Bradford and Markwell)
were applied to evaluate the protein content of samples of UHT whole milk deliberately adulterated
with melamine, ammonium sulphate or urea, which can be used to defraud milk protein and whey contents.
Compared with the Kjeldahl method, the response of the spectrophotometric methods was unaffected
by the addition of the nitrogen compounds to milk or whey. The methods of Bradford and
Markwell were most robust and did not exhibit interference subject to composition. However, the simultaneous
interpretation of results obtained using these methods with those obtained using the Kjeldahl
method indicated the addition of nitrogen-rich compounds to milk and/or whey. Therefore, this work
suggests a combination of results of Kjeldahl and spectrophotometric methods should be used to screen
for milk adulteration by these compounds.